Method and apparatus for continuous readout of fabry-perot fiber optic sensor

ABSTRACT

A pressure measurement system and method are described. The system uses a tunable laser and a Fabry-Perot sensor with integrated transducer. A detector senses the light modulated by the Fabry-Perot sensor. A signal conditioner, which can be located up to 15 km away, then uses the detector signal to determine the displacement of the diaphragm, which is indicative of pressure exerted against the diaphragm. Use of a temperature sensor to generate a signal, fed to the signal conditioner, to compensate for temperature is also contemplated.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/105,651 to Berthold et al. filed on Apr. 14, 2005, and titled METHODAND APPARATUS FOR CONTINUOUS READOUT OF FABRY-PEROT FIBER OPTIC SENSOR,which claims the benefit of priority of U.S. Provisional PatentApplication No. 60/562,430, filed Apr. 15, 2004, which are both herebyincorporated in their entirety by reference.

TECHNICAL FIELD

The present invention relates to a Fabry-Perot fiber optic sensor and,more particularly, to a high finesse Fabry-Perot sensor and tunablelaser combination system for measuring small displacements.

BACKGROUND

Low finesse Fabry-Perot interferometers have a reflectivity of approx.50% or less. Such interferometers have been used in sensors in order toprovide an indication as to the displacement of a gap between tworeflective surfaces. For example, U.S. Pat. No. 5,276,501 to McClintocket al. describes a low-finesse Fabry-Perot fiber optic sensor that usesa laser of limited tuning range. The laser itself is tuned viatemperature control to operate at two different wavelengths. Notably,reflectivity of the sensor is in the range of 4%, and the readout fromthe sensor is approximated as a two beam interferometer so that the gaplength is a function of wavelength difference and the interference fromthe additional multiple reflected beams is neglected. While this type oftwo-beam interferometer approximation may serve the purposes of theMcClintock patent, the inventors have accounted for the fact thatinterference patterns from Fabry-Perot interferometers are not periodic.Consequently, the teachings of the McClintock patent do not apply toFabry-Perot sensors in the art, especially with respect to themethodology used to perform the calculation of gap length.

Notably, both with respect to the McClintock patent and the other priorreferences known to the inventors, the range of gaps measurable by priorart laser-based Fabry-Perot sensors were limited in practice by thewavelength and tuning range of the laser. Other errors in such systemsresulted from laser instabilities and inability to precisely measure andcontrol laser wavelength.

Given the above limitations and shortcomings of the prior art, a systemthat is capable of measuring absolute values and monitor small changesin gaps in real time (i.e., at fast scan rates exceeding 2 Hz) would bewelcome by the industry. Moreover, a high-finesse system to enableaccurate calculations according to true Fabry-Perot equations is needed.

SUMMARY

The present invention addresses the aforementioned needs within theindustry by providing an accurate system using a tunable laser lightsource and a Fabry-Perot sensor configured as a transducer. TheFabry-Perot sensor receives and modulates the laser light, and the lightis tunable over a range of frequencies. The modulated light is thendetected via an InGaAs element (or similar detector means sensitive tothe selected wavelengths of the laser). The detector generates a signalbased upon the intensity ratio of the modulated laser light, and thedetector signal is fed to the signal conditioner. To insure accuracy,the temperature of the Fabry-Perot sensor in the pressure transducer maybe monitored with a second Fabry-Perot temperature sensor. The detectorsignal, and optionally the temperature signal, are provided to a signalconditioner, which identifies two frequency minima in the intensityratio and generates an output indicative of displacement between thereflective surfaces of the transducer. Preferably, the frequency rangeof the tunable laser is between 187.5 THz and 200 THz (or 1500 to 1600nm in wavelength).

A method for monitoring and quantitatively measuring small displacementsin a Fabry-Perot sensor is also contemplated. The method includesinterrogating the reflective surfaces bounding a variable unknown gap inthe Fabry-Perot sensor using a frequency-tunable laser light. The laserlight is provided over a range of frequencies, and the intensity of thelaser light modulated by the Fabry-Perot sensor is monitored to identifyat least two minima. An absolute value of the distance of the unknowngap can then be calculated from these minima. In turn, the absolutevalue for the unknown variable gap may be used to calculate and monitoran environmental parameter of the Fabry-Perot sensor—most likely thepressure applied to one of the reflective surfaces in the sensors.Optional temperature correction and/or laser pulsing can be performed toenhance the performance of the system. Additional interrogation of thesensor over a reduced range of laser frequencies for fast scanmonitoring of the variable gap is also contemplated.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of the present invention.

FIG. 2 is a cross sectional representation of the Fabry-Perot sensorused in the present invention.

FIG. 3 is a plot of the intensity of the tunable laser light versus thefrequency of the tunable laser light when the gap of the Fabry-Perotsensor is equal to 60,062 nm.

FIG. 4 is similar to the plot of FIG. 3, excepting that FIG. 4 shows theintensity versus frequency plots over a range of differing gaps.

FIG. 5 shows a plot of the Fabry-Perot sensor gap versus the frequencydifference.

FIG. 6 shows an alternate transducer design wherein the diaphragm isreplaced with a plug configuration.

DETAILED DESCRIPTION

This invention is a new approach for using fiber optic Fabry-Perotsensors to make high-resolution temperature and pressure measurements atlong distances between the sensor and the signal conditioning system.The approach requires a high power, tunable laser that can provide rapidswitching in fine increments in narrow wavelength bands withrepeatability in the infrared spectral band from 1500 to 1600 nm. Suchtunable lasers with very wide tuning range have recently becomecommercially available. By operating in the 1500 to 1600 mm spectralband where attenuation in optical fiber is very low, high resolutionpressure and temperature measurements can be made using Fabry-Perotsensors at remote distances in excess of 10000 meters with update ratesof 10 Hz.

A schematic of the invention 10 is shown in FIG. 1. Infrared light fromthe laser L is injected into a multimode optical fiber (50 μm/125 μm forexample), where it passes through a power splitter and thence to twosensors S_(p) and S_(T)-one for pressure and one for temperature,respectively. Provided the tuning range of the laser is wide enough,then each sensor S_(p), S_(T) may be interrogated at two differentwavelength bands within the tuning range of the laser L. If not, thenseparate tunable lasers with different tuning ranges may be used.Infrared light is reflected from the sensors S_(p), S_(T) back to thedetector D₁ where the light signal is converted to a photocurrent andamplified for processing in a signal conditioner (not shown) connectedto the detector. The second Fabry-Perot temperature sensor S_(T) isprovided to track the temperature of the Fabry-Perot pressure sensor.The output of the temperature sensor S_(T) can be used to correct thepressure sensor output for temperature dependent changes in the pressuresensor gap S_(p).

By way of example, the Fabry-Perot pressure sensor S_(p) is shown inFIG. 2, specifically configured as a diaphragm-type pressure transducerS_(PD). As known in the art, the general pressure sensor S_(P) may beconfigured as a transducer without a diaphragm in other ways, as furtherdescribed in FIG. 6 below. Infrared light from the tunable laser sourceis transmitted to the Fabry-Perot sensor through an optical fiber F. TheFabry-Perot sensor S_(PD) consists of two reflective surfaces 12, 14separated by a gap G. The first reflector 12 may be the end of the fiberwith a reflective coating or a separate window with reflective coating.In either case, the first reflector 12 is separated from the pressurediaphragm 16 and the second reflector 14 by a gap distance G, which isequal to 80 μm when no pressure is applied for the preferred embodiment.Also, preferably the first reflector 12 is coated with a highreflectance (R=99%) dielectric coating and the second reflector 14 iscoated with gold (R=98.5%). Together, the two parallel reflectors 12, 14separated by gap G comprise a high finesse Fabry-Perot (F-P)interferometer.

Infrared light reflected from the F-P cavity and gap G returns to thesignal conditioner (see FIG. 1) where it is detected by the photodiodedetector D₁. The detector material is InGaAs, which is sensitive in theinfrared wavelength band of interest (1500-1600 nm). The pressuretransducer S_(PD) may be configured as a circular steel (e.g.Inconel-718) plate (diaphragm) welded around the circumference of theplate to the steel sensor body. When external pressure is applied to thediaphragm 16, it deflects toward the first reflector 12 and the gap Gdecreases. The radius and thickness of the pressure diaphragm 16 arechosen so that stresses that result are much less than the yieldstrength of the material. Under these conditions, the deflection d ofthe center of the diaphragm 16 is a linear function of applied pressureP give by the equation:

d=0.2(Pr ⁴)/(Et ³)  (1)

where r is the diaphragm radius, t is the diaphragm thickness, and E isYoung's modulus of the diaphragm material. For a typical working designat P=20000 psi:

d=8.2×10⁻⁴ inch(21 μm)

where r=0.3 inch, t=0.105 inch, and E=29×10⁶ psi.

The maximum stress S is given by the equation:

S=0.8(Pr ²)/t ²  (2)

For the conditions given above, the stress S=1.3×10⁵ psi.

The infrared light intensity reflected back to the signal conditionerfrom the F-P cavity is modulated as the diaphragm deflects and the gap Gchanges. The ratio of the incident-to-reflected intensity I_(R) is afunction of both the laser frequency and the gap G and is given by

$\begin{matrix}{{I_{R}\left( {v,G} \right)} = \frac{F\; {\sin^{2}\left\lbrack {\left( {2\; \pi \; {vG}} \right)/c} \right\rbrack}}{1 + {F\; {\sin^{2}\left\lbrack {\left( {2\; \pi \; {vG}} \right)/c} \right\rbrack}}}} & (3)\end{matrix}$

where c=λν is the velocity of light, ν=1.93×10¹⁴ Hz is the frequency ofthe infrared light, λ=1550×10⁻⁹ m (1550 nm) is the wavelength, G is theFabry-Perot gap distance between the first and second reflectors,F=4R/(1−R)², and R=(R₁R₂)^(1/2) is the composite reflectance of fiberend (R₁) and diaphragm (R₂).

For illustration purposes in the remaining FIGS. 3 and 4, a compositereflectance of R=30% is assumed, although in the preferred embodimentR>99%. Shown in FIG. 3 is a plot of the intensity ratio I_(R)(ν,G) for asingle gap G=60.062 μm. Notably, such an intensity ratio can begenerated by normalizing light L provided to sensor S_(P) (and S_(T), ifappropriate), preferably through the use of detector D₂. Shown in FIG. 4is a plot of the intensity ratio I_(R)(ν,G) for various gaps. Each curvein FIG. 4 represents a different gap. As in FIG. 3 for any given gap G,the reflected intensity ratio measured by the photodiode D₁ oscillatesthrough maxima and minima as the laser frequency is tuned through itsrange. It is important to note from FIG. 4 that for any given gap, theplot of intensity ratio versus frequency is unique. Although thefunction in Equation 3 is oscillatory, the period is not repetitive,which means that the spectrum at some gap Ga does not overlay any otherspectrum for any other gap Gn. Thus, measurement of the separation ofthe minima or maxima in frequency space uniquely determines the gap towithin the system resolution.

Significantly, the inventors were the first to identify and exploit thisvariation in the intensity ratio versus frequency, as described inEquation 3. Previous methods had presumed this dependence wasrepetitive. Consequently, these previous methods could not achieve thelevel of precision for absolute, quantitative measurements attained bythe present invention. Moreover, these previous systems could notachieve the fast scan monitoring performed by the present invention.

To maximize the resolution of the system, it is important to match therange of gaps with the tuning range of the laser. For example, given alaser with a tuning range of 20 nm, and a transducer with starting gapat 0 psi pressure of 80 μm, then at 20000 psi pressure, the transducershould be designed to deflect 20 μm and the deflection range is 80 μm to60 μm. It is necessary that for all gaps in the range 60 to 80 μm, theremust be at least two minima in the F-P modulated spectrum (see FIG. 3)within the laser tuning range. The minimum length of the gap depends onthe laser operating wavelength and tuning range. For a given wavelength,the wider the tuning range the shorter the minimum allowed gap may be.

Note that the radius and thickness of the diaphragm 16 (as illustratedin the example above, a flat Inconel-718) can be chosen so that atpre-determined deflection distance (and its resulting the maximumstress, S) will be well below the yield strength of the material.Selection of a low stress is also significant because it provides a veryrepeatable pressure sensor with little or no hysteresis. As used here,hysteresis refers to the graph of sensor gap versus pressure. Ifhysteresis is present, the gap will follow two different paths—one pathwhen the pressure increases and a different path when the pressuredecreases.

An additional source of non-repeatability occurs when the stress in thediaphragm approaches the yield point of the material. When this occurs,the sensor will not produce repeatable results and will needrecalibration. Thus, it is desirable to design the transducer S_(PD) sothat the stress never approaches the yield point and for this reason,alternate transducer designs would be of great value.

An alternate for sensor S_(P) is shown as sensor S_(PS) in FIG. 6. Thetransduction mechanism is created by the compression of an tubularsleeve 20 with a plug 22 in one end. Sleeve 22 is also fitted around thetransducer body 24. Reflective surfaces can be provided on fiber F andplug 22, respectively, as discussed above. With this design there is nobending which occurs in the diaphragm design S_(PD). The resultingstress is a fraction of the stress in a diaphragm and results in a morerepeatable and durable transducer/sensor.

In the example shown with a tunable laser that operates over thewavelength range 1500 to 1600 nm (which corresponds to a frequency rangeof 200 THz to 187.5 THz, respectively speaking), it is necessary todesign both the pressure and temperature sensors so the minimum gap isapproximately 60 um. For all gaps in the range, there must be at leasttwo minima in the F-P modulated spectrum (see FIG. 3) within the lasertuning range, and the minimum length of the gap will depend on thelaser's operating wavelength and tuning range. For any given wavelength,a wider tuning range results in a shorter minimally-allowable gap. Laterwe define an algorithm which determines the gap from the measured dataand this algorithm requires that for all gaps there exist at least twominima in the F-P modulated spectrum over the tuning range.

Consider the well-known relationship for a Fabry-Perot (reference Bornand Wolf, Principles of Optics) with mirror separation G:

Δν=c/2G  (4)

where ν is the optical frequency at wavelength λ and the velocity oflight c=λν. The symbol Δ signifies a small change in the frequency ν,where Δν=ν₂−ν₁. It follows from the velocity of light that

Δν/ν=—Δλ/λ  (5)

where ν is the light frequency and λ the wavelength. The minus signsimply means that as the frequency increases the wavelength decreases.Consider a laser with an operating frequency between 192.3 THz (λ=1560nm) and 197.5 THz (λ=1519 nm). The laser frequency is tunable withtuning range Δν=5.2×10¹² Hz. Note that 1 THz=10¹² Hz. The laser is tunedin a step-wise manner and covers the range in 40000 steps where eachstep is given by the resolution element δν=1.3×10⁸ Hz. The symbol δsignifies a much smaller change than the symbol Δ, but the expression inEquation 5 continues to hold. δλ, the resolution element in wavelengthis calculated as:

δλ=(1.3×10⁸)(1540×10⁻⁹)/(1.95×10¹⁴)  (6)

which equals 1×10⁻¹² meter, i.e., 1 pm.

Equation 4 defines the spacing between the minima or spacing between themaxima plotted in FIGS. 3 and 4. Note that for any curve plotted in FIG.4, the spacing of the maximum and minimum is unique. Consider a laserwith a 5.2 THz tuning range that is shining on the Fabry-Perot sensorgap (see FIG. 1). Tune the laser over its range 192.3 THz to 197.5 THzas indicated in FIG. 3. Two minima and two maxima in the reflected lightintensity are observed. A precise measurement of the spacing Δν betweenthe minima defines the gap G. Several examples are provided in Table 1.

TABLE 1 Fabry-Perot gap as determined from measurement of Δν ν₂ THz ν₁THz Δν = c/2 G THz G (nm) 195.3 193.3 2.00 75000 194.9 192.8 2.10 71429194.8 192.5 2.30 65217 194.8 192.4 2.40 62500 195.99 193.5 2.49 60241

As long as there are at least two minima in the intensity ratio that areobserved when the laser is tuned over its range, it is always possibleto measure the gap G uniquely. A calibration plot of sensor gap versusΔν is shown in FIG. 5.

The smallest change in the gap that can be measured is determined fromEquations 4 and 5. Consider the last case in Table 1 where G=60241 nmand ν₂=195.99 THz. Calculate δG corresponding to the resolution elementδν=1.3×10⁸ Hz.

δG=G(δν/ν)  (8)

Using the parameters above, δG=60241(1.3×10⁸)/(195.99×10¹²)=0.04 nm.

For a pressure range of 20000 psi and a diaphragm deflection range of20000 nm (gap range 80 μm to 60 μm), a deflection resolution of 0.04 nmequates to a pressure resolution of 0.04 psi.

The following specifications are acceptable for the tunable laser of thepresent invention: tunable laser scans 40,000 steps in 10 sec (and canalso scan 400 steps in 0.1 see); operating scan range is 192.3 THz to197.5 THz in Laser 1; operating scan range is 186.8 THz to 192 THz inLaser 2; step size is 1 pm/step in wavelength space or 130 MHz/step infrequency space (c=λν, where c is velocity of light, λ is laserwavelength and ν is laser frequency). Additionally, Sensor gap (G) rangeis 60000 nm to 80000 nm, and the corresponding pressure range is 20000psi to 0 psi. A dither operation enables tracking of a minimum in theintensity ratio.

An algorithm that details a step-by-step method to determine the size ofgap G, and thus the applied pressure (or some other environmentalparameter associated with the Fabry-Perot sensor S_(P)), using thetunable laser L is as follows: (1) Interrogate the pressure sensor.Perform 40,000 step scan. Find the frequency minima ν₁ and ν₂. Store thestep numbers and values of ν₁ and ν₂. Calculate Δν=ν₁−ν₂. Calculate Ga(Ga=c/(2Δν), where Ga is the gap and c is the velocity of light). Notethat the location of the minima are determined to 1 pm out of 40,000 pm.The difference in the minima is known to 2 pm. Thus, the gap Ga is knownto 80 pm and the pressure is known to 0.08 psi. See Equation (8). For20,000 psi range, the pressure is determined to one part in 250,000; (2)once the gap is known, the laser is tuned to the frequency minimumnearest the center of the range and laser frequency scan range ischanged to 400 steps per 0.1 sec. In this mode, small changes indiaphragm deflection (pressure) can be tracked at high speed. Thepressure update rate in this fast scan mode is 10 Hz; (3) on a periodicbasis, repeat step 1; (4) on a periodic basis, interrogate thetemperature sensor and calculate the temperature sensor gap G_(T)(n)using a similar algorithm as in steps 1 and 2; and (5) apply temperaturecorrection factor to pressure measurement.

In summary, the sensor interrogation system consists of a tunable laserthat can provide 40,000 separate and adjacent frequency outputs over theband 192.3 THz to 197.5 THz and a photodiode to measure the lightintensity reflected from the Fabry-Perot gap in a pressure sensor. Thesystem can provide pressure measurement accuracy less than 0.1 psi. Asecond Fabry-Perot temperature sensor S_(PT) may also be provided asshown in FIG. 1, although the essence of the invention focuses on thediscovery of the non-repetitive nature of the response (as describedabove). The output of the temperature sensor can be used to correct thepressure sensor output for temperature dependent changes in the pressuresensor gap.

In long distance applications, the sensor may be 5 km, 10 km or 15 kmaway from the signal conditioner. To ensure that light from the tunablelaser reaches the sensor at the end of such long optical fiber cables,high output power is needed. An output power of 1 mW is sufficient and10 mW is typically available from tunable laser systems. Such largepower presents a fundamental problem however. When so much power isinjected into the transmission fiber, light is scattered back to thedetector. Although the percentage of light scattered back is small, thelaser power is large, so that over the first 10 meters or so of fiberlength, the amount of light back-scattered causes significant detectornoise. An optical time domain reflectometer (OTDR) experiences the sameproblem, which is why there is a dead band for the first few meters whenusing an OTDR. The large scattered light signal saturates the detector.One method to minimize or reduce the effect is to pulse the lightsource.

Light travels about 5 ns/m in optical fiber with refractive indexn=1.45. Thus it takes light about 25 μs to travel 5 km, 50 μs to travel10 km, and 75 μs to travel 15 km. If the laser is turned on and off,then for example, if the range is 10 km, the laser can be turned on for50 μs and off for 50 μs. The detector can be synchronized with the laserso that when the laser is on the detector is off and when the laser isoff the detector is on. For the 50 μs when the laser is on the lighttravels to the sensor and the detector sees no noise since it is off.For the second 50 μs, the laser is off and the detector sees infraredlight reflected from the sensor. With continuous operation in this mode,the laser light is on half the time and off half the time (50% dutycycle) and the detector noise is minimized because it is not exposed toscattered light. If the laser and detector on-time and off-time arecontinuously adjustable from 25 to 75 μs, then it is possible to adjustfor any sensor range between 5 and 15 km.

Numerous methods are available to turn the detector on and off. Theseinclude a fast shutter, electro-optic modulator, or a simple electroniccircuit to switch on and off the electric current to the laser.

1. A system for monitoring and measuring a displacement of a Fabry-Perotsensor, the system comprising: a gap of unknown variable length spanninga first reflective surface and a second reflective surface of theFabry-Perot sensor; a laser light that is tunable at a fast scan rateover a range of frequencies, where the light is provided to theFabry-Perot sensor, and the Fabry-Perot sensor modulates the light asthe light is fast scanned over the range of frequencies; a detector forgenerating a signal from a normalized intensity ratio of the modulatedlight; and a signal conditioner for identifying at least two frequencyminima in the normalized intensity ratio and generating an outputcharacteristic of the gap length between the first and second reflectivesurfaces based on the at least two identified frequency minima.
 2. Thesystem of claim 1, where the Fabry-Perot sensor comprises a high-finesseFabry-Perot sensor.
 3. The system of claim 1, where the detectorincludes an InGaAs element.
 4. The system of claim 1, further comprisinga pulsing device for pulsing the light.
 5. The system of claim 4, wherethe pulsing devise is selected from a group comprising: a fast shutter,an electro-optic modulator, and an on/off circuit for controllingcurrent provided to the laser.
 6. The system of claim 1, where the rangeof frequencies is tunable between approximately 187.5 THz andapproximately 200 THz.
 7. The system of claim 1, where at least one ofthe first and second reflective surfaces is within a pressuretransducer.
 8. A method for monitoring and measuring a displacement oftwo reflective surfaces in a Fabry-Perot sensor spanning an unknown andvariable gap comprising: tuning a laser light source over a range offrequencies to produce light; fast scanning the Fabry-Perot sensor withthe light over the range of frequencies; generating a signal from anormalized intensity ratio of light modulated by the Fabry-Perot sensor;identifying at least two frequency minima in the normalized intensityratio; generating an output characteristic of the displacement betweenthe two reflective surfaces based on the at least tow identifiedfrequency minima; and calculating an absolute value for the unknownvariable gap distance during the fast scanning of the Fabry-Perotsensor.
 9. The method of claim 8, where the absolute value for theunknown variable gap is used to calculate an environmental parameter.10. The method of claim 9, further comprising: monitoring a temperaturesensor to generate a temperature signal; and applying a temperaturecorrection factor in calculating the environmental parameter of theFabry-Perot sensor.
 11. The method of claim 10, further comprising:monitoring the light modulated by the Fabry-Perot sensor during the fastscanning to detect changes in the environmental parameter.
 12. Themethod of claim 11, further comprising pulsing the light fast scanningthe Fabry-Perot sensor.
 13. The method of claim 8, further comprisingpulsing the light fast scanning the Fabry-Perot sensor.
 14. A system formonitoring and measuring a displacement of reflective surfaces in aFabry-Perot sensor, the system comprising: a laser providing lighttunable at a fast scan rate over a range of frequencies; a gap ofunknown variable length spanning a first reflective surface and a secondreflective surface of the Fabry-Perot sensor, the Fabry-Perot sensorreceiving and modulating the fast scanned light; a temperature sensorgenerating a temperature signal characteristic of an environmentaltemperature at the Fabry-Perot sensor; a detector for generating asignal from a normalized intensity ratio of the modulated light; and asignal conditioner for identifying at least two frequency minima in thenormalized intensity ratio and generating an output characteristic ofthe displacement between the two reflective surfaces based on the atleast two identified frequency minima.
 15. The system of claim 14,further comprising a pulsing device for pulsing the light.
 16. Thesystem according to claim 15, where the pulsing device is selected froma group comprising: a fast shutter, an electro-optic modulator, and anon/off circuit for controlling current provided to the laser.
 17. Thesystem of claim 14, where the range of frequencies is betweenapproximately 187.5 THz and approximately 200 THz.
 18. The system ofclaim 14, where at least one of the first and second reflective surfacesis included within a pressure transducer.
 19. The system of claim 14,where the Fabry-Perot sensor comprises a high-finesse Fabry-Perotsensor.
 20. The system of claim 14, where the detector includes anInGaAs element.